Human granulocytic anaplasmosis (HGA; formerly human granulocytic ehrlichiosis) is an emerging and potentially deadly disease. It is the second most common tick-borne infection in endemic areas of the United States. HGA is caused by Anaplasma phagocytophilum, an obligate intracellular bacterium that colonizes a most unusual host cell - the neutrophil. Neutrophils normally play beneficial roles in microbial clearance and inflammation in response to injury or infection. Neutrophil dysfunction, as associated with inflammatory disorders, can have considerably damaging effects. A. phagocytophilum cellular adhesion to and invasion of human neutrophils are dependent on bacterial recognition of P-selectin glycoprotein ligand-1 (PSGL-1) and sialylated and fucosylated glycans on neutrophil surfaces. Using a novel assay, we further defined the A. phagocytophilum cytoadherence mechanism by identifying 2 key molecular features of PSGL-1 to which the organism binds: (i) a primary amino acid sequence found in the N-terminus of human PSGL-1 and (ii) sialyl Lewis x (sLex), a sialylated and fucosylated tetrasaccharide that modifies PSGL-1 and other selectin ligands. The bacterial protein(s) that mediate these interactions are unknown, which represents a considerable gap in our knowledge. We hypothesize that A. phagocytophilum adherence to neutrophils is dependent on 1 or more adhesins that target PSGL-1 N-terminal peptide and sLex. Identifying the cognate adhesins will be integral to developing strategies for disrupting the interaction of A. phagocytophilum with the surface of its host cell. The objectives of this proposal are to identify and characterize the A. phagocytophilum adhesin(s) that mediate cytoadhesion to human neutrophils. The specific aims are: (1) to isolate putative adhesins based on their affinities for PSGL-1 and sLex and identify them using proteomic and molecular methods, (2) to functionally characterize the adhesin candidates using glycoconjugate and cellular binding assays. Achieving these aims will provide the initial framework for a comprehensive assessment of the adhesin(s)' contributions to A. phagocytophilum infection. Additionally, it will fill significant voids in our understanding of A. phagocytophilum pathogenesis and bacterial adhesion strategies. Furthermore, it will identify novel targets for preventing and treating HGA and may pave the way for development of pharmacologic inhibitors of cellular adhesion events associated with disease.